Cathode for electron discharge device



Oct. 24, 1961 c. E. HORNER ETAL CATHODE FOR ELECTRON DISCHARGE DEVICEFiled May 22, 1959 Fig.|.

INVENTORS Clifford E. Homer and Gene R. Feoster.

Y ATTdRNEY WITNESSES -Q RGAL WW fi a y Filed May 22, 1959, Ser. No.815,173 6 Claims, 3- 3 and Gene R. Feaster, Ele t i of This inventionrelates to electron discharge devices, and more particularly toimprovements in thermionic electron emitting cathodes for such devices.

In the conventional indirectly heated type cathode the usualeonstruction of the cathode-heater assembly is a nickel sleeve having anexternal oxide coating and a filamentary heater coated with. arefractory insulation provided in the interior region of the sleeve. Incertain electron discharge tubes, such as the so-called electron guns inkinescopes and camera tubes, the cathode normally consists of a cappednickel alloy cylinder. The cap is coated with emissive oxides such thatelectrons are emitted in an end-fire manner from the external end of thecathode cylinder. The heater in this structure is customarily comprisedof a non-inductive, reverse helix of tungsten wire coated with Alundurnacrystalline form of aluminum oxide. The heater is positioned within thecathode cylinder.

It has been found in commercial practice that when such a heater cathodeconstruction as described above is subjected to vibration duringoperation failures frequently occur as a result of breakage of thetungsten wire. It is found upon examination that the heater wire hasbecome excessively brittle as a result of operation while subjected tovibration. It has also been found that the heater wire becomes extremelybrittle after a given operation time and that severe jars will oftencause the wire to break, opening the cathode heater circuit andsometimes causing shorts between the cathode heater and the cathodesleeve. Tests were conducted to determine the cause of this breakage andit was found that there was a definite relationship between the lengthof of time the tube was vibrated and the amount of brittleness found inthe heater wire. It was found that heaters from tubes which werevibrated for a shorter period of time were in general less brittle. Itwas also found that when the tubes were vibrated with the heaters cold,there was, in all cases, no evidence of brittleness. It is believed thatthe relative motion of the nickel sleeve and the loosely supportedheater within the nickel sleeve causes the hard Alundum coating on theheater wire to abrade nickel material from the inner surface of thenickel sleeve. From spectrographic analysis, it was found that nickelwas transferred from the nickel sleeve to the tungsten heater. When thetungsten heater, containing the spectrographically traces of nickel onthe surface, was subsequently heated above approximately 1200" C., thetungsten heater became extremely brittle. The nickel material entrappedin the Alundum will be heated to the temperature of the heater,approximately 1300 C. At this temperature the nickel has a vaporpressure of the order of one micron, consequently the tungsten isexposed to excessive amounts of nickel vapor. This is especially sowhere the Alundum coating is cracked. The cathode sleeve contributesvery little nickel vapor directly since at cathode temperature about 800C., the vapor pressure of nickel is less than mm. of mercury.

It is accordingly an object of this invention to provide an improvedindirectly heated cathode assembly.

It is another object to provide an improved indirectly heated cathodethat will withstand substantial vibration.

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It is another object to provide an improved indirectly heated cathode inwhich the tungsten heater does not become brittle even after longlife'and high temperature operation.

These and other objects are effected by our invention as will beapparent from the following description taken in accordance with theaccompanying draw: ing throughout which like reference charactersindicate like parts, and in which:

FIG. 1 is a view partly 1n section embodying the teach ings of ourinvention; and

FIG. 2 is a view partly in section, illustrating another cathodestructure embodying the teaching of our invention.

Referring in detail to 'FIG. 1, there is illustrated a so-calledend-fire type cathode in which the electrons are emitted from theexternal end of the cathode sleeve. The cathode structure showncomprises a tubular sleeve member 10 of a suitable material having ahigh melting point, a low vapor pressure and non-embrittlement influenceon tungsten such as molybdenum and base alloys thereof. The sleevemember 10 has one end closed with an end portion 12 also of molybdenum.The molybdenum may be machined or drawn to desired shape. The molybdenummay be of the type suitable for use in electronic tubes. A cap 14 of asuitable core material such as nickel with suitable reducing agents(standard nickel cathode grade) is secured to the closed end of thetubular sleeve 10 by spot Welding. A coating 16 of a suitable electronemissive material such as barium-strontium carbonate is provided on theexterior surface of the nickel cap 14. The coating 16 may be depositedby any suitable process such as spraying. Positioned within the tubularsleeve is a heater member 20. The heater 2% consists of a noninductivereverse helix wound wire 22 of a suitable material such as tungstenhaving a coating 24 of a suitable material such as Alundum. A supportmember 30 is also illustrated secured to the outer portion of thetubular sleeve 10 for supporting the cathode within an electrode as iswellknown in the art.

In the structure illustrated in FIG. 1, it can be seen that the heater20 during vibration can abrade only the molybdenum sleeve 10. Themolybdenum material which may be transferred to the Alundum coating 24will not vaporize at the operating temperature of the heater 2% and willnot embrittle the tungsten in any case. -In the specific embodimentsshown and described only molybdenum was indicated as a suitable materialfor the sleevemember 10. Other suitable materials are tungsten andtantalum and base alloys thereof. These materials have a low vaporpressure and their vapors are not damaging to tungsten.

In FIG. 2 there is illustrated a cathode structure for use in aconventional type receiving tube. This type of cathode consists of atubular member with an electron emissive material on the externalsurface. A heater is provided within the tubular member and the cathodemay be supported at each end.

In the specific embodiment shown a sleeve member 40 is provided ofnickel similar to the material used in the end cap member 14 of FIG. 1.An oxide coating 42. of a material similar to the coating 16 in FIG. 1is provided. on a portion of the external surface of the sleeve 40. Thecoating 42 may be applied in a similar manner to that described withrespect to FIG. 1. On the inner surface of the sleeve 40 is provided acoating or a liner 44 of a material similar to that described withrespect to the member 12 of FIG. 1. If the layer 44 is a liner, then itis inserted into the sleeve 42 and secured to the nickel sleeve by spotWelding. The heater 20 is provided within the inner region defined bythe sleeve 3 42 and is shielded from the sleeve 40 by the member 44.

The layer 44 may also be provided as a coating of a suitable materialsuch as molybdenum, tungsten, tantalum, applied by methods known in theart.

Thus it can be seen that our invention provides the cathode structurewhich will withstand vibration and necessary processing and operatingtemperatures Without embrittlement of the tungsten heater element by thenickel material.

While we have shown our invention only in two forms, it will be obviousto those skilled in the art that it is not so limited but is susceptibleof various other changes .and modifications without departing from thespirit and 2. An indirectly heated cathode comprising a tubular memberof a material selected from the group consisting of molybdenum, tungstenand tantalum and base alloys thereof, an end cap of cathode grade nickelon one end of said tubular member, an electron emissive oxide coating onthe surface of said end cap and a heater member containing tungstenmaterial provided in the interior region of said tubular member.

3. An indirectly heated cathode comprising a tubular sleeve member of amaterial selected from the group consisting of molybdenum, tungsten andtantalum and base alloys thereof, a second sleeve member of cathodegrade nickel surrounding said first sleeve member, an electron emissivecoating on the external surface of said second sleeve member and aheater element containing tungsten material provided within the interiorregion of said first sleeve member.

4. An indirectly heated cathode comprising a tubular sleeve membercontaining nickel, a layer of a material selected from the groupconsisting of molybdenum, tungsten and tantalum and base alloys thereof,an electron emissive coating on the external surface of said nickelsleeve and a heater element containing tungsten material provided withinthe interior region of said sleeve.

5. An indirectly heated cathode comprising a tubular sleeve membercontaining nickel, the inner surface of said nickel sleeve having amaterial selected from the group consisting of molybdenum, tungsten andtantalum and base alloys thereof coated on said nickel sleeve, anelectron emissive coating on the external surface of said nickel sleeveand a heater element containing tungsten material provided within theinterior region of said sleeve.

6. An indirectly heated cathode comprising a member containing nickelhaving an electron emissive coating on one surface thereof, a heaterelement containing tungsten material disposed facing a side of saidnickel member remote from said electron emissive coating and a layer ofa material selected from the group consisting of molybdenum, tungsten,tantalum and the base alloys thereof disposed between said nickel memberand said heater element.

No references cited.

